Mehul Motani

Cross-layer design: a survey and the road ahead

Of late, there has been an avalanche of cross-layer design proposals for wireless networks. A number of researchers have looked at specific aspects of network performance and, approaching cross-layer design via their interpretation of what it implies, have presented several cross-layer design proposals. These proposals involve different layers of the protocol stack, and address both cellular and ad hoc networks. There has also been work relating to the implementation of cross-layer interactions. It is high time that these various individual efforts be put into perspective and a more holistic view be taken. In this article, we take a step in that direction by presenting a survey of the literature in the area of cross-layer design, and by taking stock of the ongoing work. We suggest a definition for cross-layer design, discuss the basic types of cross-layer design with examples drawn from the literature, and categorize the initial proposals on how cross-layer interactions may be implemented. We then highlight some open challenges and new opportunities for cross-layer design. Designers presenting cross-layer design proposals can start addressing these as they move ahead.

PeopleNet: engineering a wireless virtual social network

People often seek information by asking other people even when they have access to vast reservoirs of information such as the Internet and libraries. This is because people are great sources of unique information, especially that which is location-specific, community-specific and time-specific. Social networking is effective because this type of information is often not easily available anywhere else. In this paper, we conceive a wireless virtual social network which mimics the way people seek information via social networking. PeopleNet is a simple, scalable and low-cost architecture for efficient information search in a distributed manner. It uses the infrastructure to propagate queries of a given type to users in specific geographic locations, called bazaars. Within each bazaar, the query is further propagated between neighboring nodes via peer-to-peer connectivity until it finds a matching query. The PeopleNet architecture can overlay easily on existing cellular infrastructure and entails minimal software installation. We identify three metrics for system performance: (i) probability of a match, (ii) time to find a match and (iii) number of matches found by a query. We describe two simple models, called the swap and spread models, for query propagation within a bazaar. We qualitatively argue that the swap model is better with respect to the performance metrics identified and demonstrate this via simulations. Next, we compute analytically the probability of match for the swap model. We show that the probability of match can be significantly improved if, prior to swapping queries, the nodes exchange some limited information about their buffer contents. We propose a simple greedy algorithm which uses this limited information to decide which queries to swap. We show via simulation that this algorithm achieves significantly better performance. Overall our results demonstrate that PeopleNet, with its bazaar concept and peer-to-peer query propagation, can provide a simple and efficient mechanism for seeking information.

On The Han–Kobayashi Region for theInterference Channel

In this correspondence, we derive a simplified description of the Han-Kobayashi rate region for the general interference channel. Using this result, we establish that the recently discovered Chong-Motani-Garg rate region is a new representation of the Han-Kobayashi region. Moreover, a tighter bound for the cardinality of the time-sharing auxiliary random variable emerges from our simplified description.

Adaptive contact probing mechanisms for delay tolerant applications

In many delay tolerant applications, information is opportunistically exchanged between mobile devices who encounter each other. In order to effect such information exchange, mobile devices must have knowledge of other devices in their vicinity. We consider scenarios in which there is no infrastructure and devices must probe their environment to discover other devices. This can be an extremely energy consuming process and highlights the need for energy conscious contact probing mechanisms. If devices probe very infrequently, they might miss many of their contacts. On the other hand, frequent contact probing might be energy inefficient. In this paper, we investigate the trade-off between the probability of missing a contact and the contact probing frequency. First, via theoretical analysis, we characterize the trade-off between the probability of a missed contact and the contact probing interval for stationary processes. Next, for time varying contact arrival rates, we provide an optimization framework to compute the optimal contact probing interval as a function of the arrival rate. We characterize real world contact patterns via Bluetooth phone contact logging experiments and show that the contact arrival process is self-similar. We design STAR, a contact probing algorithm which adapts to the contact arrival process. Via trace driven simulations on our experimental data, we show that STAR consumes three times less energy when compared to a constant contact probing interval scheme.

Analysis and implications of student contact patterns derived from campus schedules

Characterizing mobility or contact patterns in a campus environment is of interest for a variety of reasons. Existing studies of these patterns can be classified into two basic approaches - model based and measurement based. The model based approach involves constructing a mathematical model to generate movement patterns while the measurement based approach measures locations and proximity of wireless devices to infer mobility patterns. In this paper, we take a completely different approach. First we obtain the class schedules and class rosters from a university-wide Intranet learning portal, and use this information to infer contacts made between students. The value of our approach is in the population size involved in the study, where contact patterns among 22341 students are analyzed. This paper presents the characteristics of these contact patterns, and explores how these patterns affect three scenarios. We first look at the characteristics from the DTN perspective, where we study inter-contact time and time distance between pairs of students. Next, we present how these characteristics impact the spread of mobile computer viruses, and show that viruses can spread to virtually the entire student population within a day. Finally, we consider aggregation of information from a large number of mobile, distributed sources, and demonstrate that the contact patterns can be exploited to design efficient aggregation algorithms, in which only a small number of nodes (less than 0.5%) is needed to aggregate a large fraction (over 90%) of the data.

Investigating network architectures for body sensor networks

The choice of network architecture for body sensor networks is an important one because it significantly affects overall system design and performance. Current approaches use propagation models or specific medium access control protocols to study architectural choices. The issue with the first approach is that the models do not capture the effects of interference and fading. Further, the question of architecture can be raised without imposing a specific MAC protocol. In this paper, we first evaluate the star and multihop network topologies against design goals, such as power and delay efficiency. We then design experiments to investigate the behavior of electromagnetic propagation at 2.4 GHz through and around the human body. Along the way, we develop a novel visualization tool to aid in summarizing information across all pairs of nodes, thus providing a way to discern patterns in large data sets visually. Our results suggest that while a star architecture with nodes operating at low power levels might suffice in a cluttered indoor environment, nodes in an outdoor setting will have to operate at higher power levels or change to a multihop architecture to support acceptable packet delivery ratios. Through simple analysis, the potential increase in packet delivery ratio by switching to a multihop architecture is evaluated.

Cooperative Asynchronous Multichannel MAC: Design, Analysis, and Implementation

Medium access control (MAC) protocols have been studied under different contexts for decades. In decentralized contexts, transmitter-receiver pairs make independent decisions, which are often suboptimal due to insufficient knowledge about the communication environment. In this paper, we introduce distributed information sharing (DISH), which is a distributed flavor of control-plane cooperation, as a new approach to wireless protocol design. The basic idea is to allow nodes to share control information with each other such that nodes can make more informed decisions in communication. This notion of control-plane cooperation augments the conventional understanding of cooperation, which sits at the data plane as a data relaying mechanism. In a multichannel network, DISH allows neighboring nodes to notify transmitter-receiver pairs of channel conflicts and deaf terminals to prevent collisions and retransmissions. Based on this, we design a single-radio cooperative asynchronous multichannel MAC protocol called CAM-MAC. For illustration and evaluation purposes, we choose a specific set of parameters for CAM-MAC First, our analysis shows that its throughput upper bound is 91 percent of the system bandwidth and our simulations show that it actually achieves a throughput of 96 percent of the upper bound. Second, our analysis shows that CAM-MAC can saturate 15 channels at maximum and our simulations show that it saturates 14.2 channels on average, which indicates that, although CAM-MAC uses a control channel, it does not realistically suffer from control channel bottleneck. Third, we compare CAM-MAC with its noncooperative version called UNCOOP, and observe a throughput ratio of 2.81 and 1.70 in single-hop and multihop networks, respectively. This demonstrates the value of cooperation. Fourth, we compare CAM-MAC with three recent multichannel MAC protocols, MMAC, SSCH, and AMCP, and find that CAM-MAC significantly outperforms all of them. Finally, we implement CAM-MAC and UNCOOP on commercial off-the-shelf hardware and share lessons learned in the implementation. The experimental results confirm the viability of CAM-MAC and the idea of DISH.

MAX: human-centric search of the physical world

MAX is a system that facilitates human-centric search of the physical world. It allows humans to search for and locate objects as and when they need it instead of organizing them a priori. It provides location information in a form natural to humans, i.e., with reference to identifiable landmarks (e.g., on the dining table) rather than precise coordinates. MAX was designed with the following objectives: (i) human-centric operation, (ii) privacy, and (iii) efficient search of any tagged object. In the system, all physical objects, from documents to clothing, can be tagged and people locate objects using an intuitive search interface. To make search efficient, MAX adopts a hierarchical architecture consisting of tags (bound to objects), sub-stations (bound to landmarks) and base-stations (bound to localities). Tags can be marked as either public or private, with private tags searchable only by the owner. MAX also provides for privacy of physical spaces.MAX requires minimal initial configuration, and is robust to reconfiguration of the physical space. To optimize system performance, we present a methodology to design energy and delay optimal query protocols for a variety of device choices. We have implemented MAX using Crossbow motes and conducted user trials in a 5m by 5m cluttered office. The user feedback was positive, demonstrating the feasibility of MAX for human-centric search. We contend that a MAX-like search system will enable sharing (e.g., books on a college campus) and trading (e.g., buying and selling used books) of physical resources, and will be the engine for a host of new applications.

Inter-User Interference in Body Sensor Networks: Preliminary Investigation and an Infrastructure-Based Solution

Inter-user interference is the interference in communication when several Body Sensor Networks (BSNs) operate in the same vicinity. As BSN users congregate in an area, interference due to concurrently communicating BSNs will increase, resulting in poor performance. In this paper, we conduct a preliminary investigation of the impact of inter-user interference and investigate its behavior with respect to parameters such as number of networks and the rates at which these networks communicate. Inter-user interference, is seen to reduce Packet Delivery Ratio by almost 35% in cases of 8 or more high-rate networks operating in the same location. We also propose a system to mitigate the adverse effects of inter-user interference. Our solution uses a fixed network infrastructure to monitor and identify BSNs that are likely to interfere with each other. The network then recommends changes to the BSN protocol to lessen interference between them. We also implement an instance of our system using a Wireless Sensor Network (WSN) infrastructure to reduce interference. The system is shown to significantly decrease the impact of inter-user interference.

To Hop or Not to Hop: Network Architecture for Body Sensor Networks

As Body Sensor Networks (BSNs) advance to fulfill the promise of continuous, non-intrusive, remote monitoring of patients, it is important that we achieve efficient communication between energy constrained on-body sensors. An important design choice which has significant impact on achieving this goal is the network architecture. Star architecture has been the natural choice for BSNs due to the short distances between the nodes. In this paper, we revisit this choice by quantitatively studying architecture choices using data from experiments conducted by deploying nodes operating at the 2.4 GHz band on actual human volunteers. We compare the star and multihop architectures to highlight their respective performance characteristics. In particular, we use our data to construct multihop networks with routes that maximize end-to-end Packet Delivery Ratio (PDR) and routes that minimize the average number of retransmissions. Since BSNs span an entire spectrum of applications, each with its unique constraints and requirements, there is no solution that is optimal for all applications. Instead, we show the performance across a variety of metrics and the trade-offs that are achievable. We see that a multihop network minimizing retransmissions has several advantages including having better network lifetime as well as the lowest delay and energy consumption.